Fix standalone GUI close race condition and update baseview

[dfl]

Summary

• Fix race condition when closing standalone GUI window that caused crashes with "Rust cannot catch foreign exceptions" errors on macOS
• Update baseview to latest revision which includes a null pointer fix for become_first_responder on macOS (PR avoid crash when window is null in become_first_responder RustAudio/baseview#204)

Problem

When closing the standalone app via the GUI (Cmd+Q or clicking X on macOS), there was a race condition where:

1. Window::open_blocking() returns and the WrapperWindowHandler is dropped
2. This drops _editor_handle, starting editor cleanup
3. Only AFTER that do we signal the audio thread to stop
4. The audio thread may still be accessing shared resources during cleanup, causing the crash

Solution

Handle WindowEvent::WillClose in WrapperWindowHandler::on_event to:

1. Signal the audio thread to terminate
2. Wait for the audio thread to finish (with 5s timeout)
3. Then allow the window to close and the editor to be dropped

Also updates baseview from rev 579130ec to 237d323c which fixes a separate macOS crash during window initialization with newer Rust versions (1.86+).

Test plan

• Build standalone plugin with GUI (e.g., gain_gui_vizia)
• Run the standalone and verify window opens
• Close via window X button - should exit cleanly
• Verify no "foreign exceptions" crash

🤖 Generated with Claude Code